An aquarium is a container of water where pet fish live.

A aquarium can be fresh water or salt water.

Fish have been on Earth for more than 500 million years ago, since before the dinosaurs.

There are more than 24,000 different kinds of fish, which is more than the total number of amphibian, reptile, bird and mammal species added together. Fish range in size from the tiny 8 millimetre Philippine goby to the giant 16 metre whale shark. Almost half of fish live in freshwater, even though most of the Earth's water is salt.

There are three groups of fish: jawless, cartiliginous and bony:

The earliest fish were jawless fish, and today only a few species remain: lampreys and hagfish. They have a long body like that of an eel. Instead of a jawbone, they have a round, open moth called an oral disk. Some are parasites, meaning they attach their mouths to another fish, eat through their flesh and suck their blood. Hagfish can ooze slime so that predators can't get a grip on them. They feed on dead sea animals.

Bony fish are jawed fish whose skeletons are made of bone. Bony fish are found all over the world, in different kinds of water.Some live in salt water but breed in freshwater, some live in freshwater but breed in salt water.

Cartilaginous fish are jawed fish whose skeletons are made of cartilage instead of bone, and they have no ribs. Cartilage is a type of tissue that is more flexible than bone. They do not have swim bladders, so they must move continuously. Sharks, rays and sawfish are cartilaginous fish.

Body

Fish are vertebrates, meaning they are animals that have a backbone.

Getty Images

Fish are aquatic, meaning they live only in water, taking oxygen from the water through body parts called gills.

Most fish are cold-blooded, meaning their body temperature changes with water temperature, although some, such as tuna and swordfish, can control temperatures in some parts of their bodies.

Fish body covering is scales, overlapping from head to tail. Fish have a special sense organ that helps them get around in murky or dark water. It is called a lateral line.

Most fish have streamlined bodies that taper at each end and are bigger in the middle. Most have a swim bladder, a body part filled with air that allows the fish to stay afloat even when it is not moving. Most fish have flexible fins, usually a dorsal (back) fin, a caudal (tail) fin, an anal (underside and in front of tail) fin, a pair of pectoral (side) fins and a pair of ventral (below the pectoral fins, close to the abdomen) fins. These help a fish to balance and move in the water.

In some fish species, males and females look different, and in others there is little or no visible difference. There are species of fish that can 'fly' or leap from the water and glide for short distance, others can 'skip' along the surface. There are some that can climb onto rocks.

Breathing

Fish take in oxygen that has dissolved in water. They have gills, and there is one gill opening on each side of the body, covered by a hard flap of bone. Water taken in through the mouth passes through the gills, where oxygen is absorbed before the water exits through the gill flaps.

Some fish eat other fish. Getty Images

Diet

Depending on its size and habitat, a fish diet includes other fish, molluscs, water plants, algae, zooplankton, insects, water birds, turtles, frogs, snakes and mice.

Fish as pets

Pet goldfish are a good pet to start off with. Getty Images

There are hundreds of different kinds of fish that can be kept as pets. Pet fish live in an aquarium. An aquarium can be a small bowl or tank or it can be a very large tank.

Goldfish are the most commonly kept pet fish because they live in cold water. They are freshwater fish and are a good fish to have if you are starting out. You can keep them in an aquarium or outside in a pond. Other good freshwater fish for beginners are platties, sword tails, guppies and mollies.

Saltwater tanks

These are more difficult to manage because you have to add salt to fresh water and you must test the water with a special test kit to make sure you have the saltiness just right. Tanks must be kept clean and the water at the right temperature or the fish will get sick.

Cichlids

Cichlids (say sic-lid) are a popular freshwater aquarium fish all over the world, and can be bought in shops that sell fish for pets. Cichlids are found in Africa and Madagascar, in South and Central America, and in India.

There are thousands of species, or kinds, of cichlids, in a huge variety of sizes, shapes and colours. There are different kinds of cichlid in each of the large African lakes, particularly Lake Malawi, Lake Tanganyika and Lake Victoria. No other lake in the world has such a large variety of cichlids as Lake Malawi (sometimes called Lake Nyasa) has.

The cichlids occupy different habitats in the lakes: from the sandy shallows to the cold deep water, and amongst the rocks.

Cichlids of Lakes Malawi, Tanganyika and Victoria are a good example of species flock. Species flock is when a large group of closely related different species live in a confined area such as a lake. In order to all survive each species adapts, or changes, some of its behaviours. For example, rather than all the fish eating the same thing, some change to eat differently. That way they all survive without running out of one kind of food. The different kinds of cichlid in each lake feed in different ways so that there is enough food to go round. They have different diets, mouths and teeth. Some feed on algae, and have developed mouths and teeth that allow them to graze on algae growing on smooth rock surfaces.

They are able to lie flat on the rocks and scrape the rock while watching out for danger. Some feed on plants or other fish. Some sift the sand to find food, others crush snails.

Another adaptation in cichlids is the different ways of reproducing. Cichlids are generally mouth brooders. This means that the females keep their fertilised eggs in their mouths until the eggs hatch, for better protection in the crowded lake. Some kinds have adapted to keeping the tiny hatchlings in their mouth for a few weeks, letting them out for a swim in safe spots, then taking them into their mouth again before they get into danger.

Siamese fighting fish

Another popular aquarium fish, they are from Southeast Asia, especially Thailand and Cambodia. Siam is the ancient name for Thailand. The fish live in shallow, overgrown waters including irrigation channels and flooded rice fields. Shallow water stays warm, up to 30°C. These fish do not live in cold water.

Siamese Fighting fish are part of a fish family that all have a special breathing organ, called labyrinth, which they can use to absorb air. Labyrinth fishes that are prevented from taking air will drown. Because of their ability to breath air, labyrinth fish can survive in polluted water that does not have very much oxygen. Males are larger than females, and more colourful, with larger fins.

Siamese fighting fish life cycle

Males build a nest of bubbles among plant leaves. To make the bubbles, the fish takes up air in its mouth, coats it with saliva (spit), and spits out the bubbles, which stick together on the surface of the water. After this, he shows off to attract a female below the nest. They swim around together, and she releases eggs. He fertilises them with liquid from his body, which means that baby fish will grow inside the eggs. As the eggs float to the bottom, the male catches them in his mouth and spits them into the bubble nest. He then watches over the nest, protecting the eggs from being eaten and putting any fallen eggs back into the nest. When the eggs hatch, the little fish swim away. Sadly, the male fish, after all that work looking after the eggs, sometimes eats the baby fish.

Siamese fighting fish males are aggressive and will even attack their own reflection in a mirror. The males do not get along well with each other. You can only keep more than one male if you have a very large tank so they have enough space to form their own territories. In the wild, males have plenty of room to stay out of each other's way. In captivity, however, that aggressiveness is why the males are kept in separate tanks or in a large one with barriers.The aquarium should be thickly planted and well lit. It should be kept warm, between 23ºC and 30ºC. Floating plants will allow nest building.

Responsible pet ownership

Pet fish depend on their owners for food, protection and shelter. Before becoming a pet owner think carefully about all the things you will have to do to care for your pet responsibly.

Setting up an aquarium

Whatever kind of fish you decide to keep, when you buy fish, get advice about the size of tank to buy and whatever other equipment you need, such as filter or heater. Get advice also about how to feed and care for the kind of fish you choose. Buy the fish food they recommend for the kind of fish you get. Set up the aquarium with water, gravel and plants and let the water stand for a day to get the chemical balance right. If you have a filter, that should be running before you get the fish.

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